1. Field of the Invention
The invention relates to a system for transmitting signals over a short wave link between a central control and recording laboratory and seismic data acquisition apparatus, using one or more radio relays.
The transmission system of the invention relates more particularly to the transmission of signals between a central control system and seismic data acquisition apparatus adapted for transmitting, both by radio and by transmission cable, the seismic signals they have collected, digitized and recorded. These signals are supplied thereto by seismic sensor assemblies spaced apart along a seismic profile to be studied, in response to the echoes from the reflecting layers of the subsoil of seismic signals generated in the ground by an appropriate shock producing source.
2. Description of the Prior Art
Devices for transmitting orders or data by cable or by short wave link are described in French published patent applications Nos. 2.511.772 and 2.538.561. They are associated, on the one hand, with a plurality of acquisition apparatus each adapted for communicating with a central control and recording system by radio or by cable, which are disposed in boxes or buoys deposited on the ground at a certain distance from each other and connected to the seismic receivers.
The acquisition apparatus having collected the data delivered by the seismic sensors which are associated respectively therewith, the central control and recording system sends transmission orders successively to all the acquisition apparatus so that they transmit the collected signals. The data transmissions are carried out preferably by short wave link when the quality of the transmissions is sufficient. But when the ground where the boxes or buoys are positioned is very uneven or when obstacles (thick forest for example) prevent the correct transmission of hertzian waves from certain boxes or certain groups of particular boxes, they must be connected by a transmission cable to the control and recording system, the others being connected to the central system by ratio.
Cable connections are very reliable whatever the nature or relief of the ground but positioning thereof requires time consuming and expensive setting up and removal operations, especially in mountains or marshy regions.
The selective connection of the boxes or buoys to the common transmission cable substantially simplifies the installation face in the field of the seismic reception assembly. But this combined system sometimes has certain drawbacks in the case where, for example, the number of cases for which the satisfactory short wave links are impossible is relatively small or where it is however necessary to unwind a transmission cable, sometimes over very large distances, for connection them to the central system, while passing through zones where the short wave links are satisfactory. This case occurs for example when a part of the boxes or buoys is disposed on the same slope of a rise in the ground as the central control and recording system, and is accessible by short wave link and when another part of them is installed on the opposite slope and only accessible by means of a transmission cable. Such a cable must consequently be positioned so as to cross over the rise in the ground and allow the connection of these distance boxes to the central system.
In some cases also most of the boxes or buoys are disposed in a zone in which the radio transmissions are satisfactory but, on the other hand, accessible only with difficulty from the central control and recording system when this latter is outside said zone, which also means that a transmission cable of great length must be unwound between these boxes and the central system.
The transmission system of the invention overcomes the above mentioned drawbacks.